Arrays of Ag and Au Nanoparticles with Terpyridine - ACS Publications

Apr 10, 2017 - charge transfer (CT) transition.33−37 Lombardi et al. integrated ...... Plasmonics; John Wiley & Sons, Inc. and Science Wise Publishi...
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Arrays of Ag and Au Nanoparticles with Terpyridine- and Thiophene-based Ligands: Morphology and Optical Responses Marketa Pruskova, Veronika Sutrova, Miroslav Slouf, Blanka Vlckova, Jiri Vohlidal, and Ivana Sloufova Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b00126 • Publication Date (Web): 10 Apr 2017 Downloaded from http://pubs.acs.org on April 11, 2017

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Arrays of Ag and Au Nanoparticles with Terpyridine- and Thiophenebased Ligands: Morphology and Optical Responses Markéta Prusková1, Veronika Sutrová1, Miroslav Šlouf2, Blanka Vlčková1, Jiří Vohlídal1*, Ivana Šloufová1* 1

Charles University, Faculty of Science, Department of Physical and Macromolecular Chemistry, Hlavova 2030, 128 40 Prague 2, Czech Republic

2

Institute of Macromolecular Chemistry, Academy of Sciences of the Czech Republic, Heyrovsky Sq. 2, 162 06 Prague 6, Czech Republic *E-mail for I.Š.: [email protected], *E-mail for J.V.: [email protected]

Abstract Assembling of Ag and Au nanoparticles (NPs) into nanoparticulate arrays mediated by terpyridine (tpy), 4´-(2-thienyl)terpyridine (T-tpy) and short α,ω-bis(tpy)oligothiophene ligands has been accomplished at the interface between the Ag or Au NP hydrosol and a solution of the molecular species in dichloromethane. The relationship between the morphology and the optical responses of the arrays has been investigated by advanced methods of the TEM (transmission electron microscopy) image analysis and surface plasmon extinction (SPE) spectra. It has been established that the size of islands of closely spaced NPs rather than the average interparticle distance affects the extent of delocalization of the surface plasmon excitations and thus also the SPE spectra. Furthermore, the structure of surfaceadsorbate complexes formed in these arrays has been investigated by SERS spectral measurements carried out as a function of the excitation wavelength. Photoinduced charge transfer (CT) transitions from the neutral Ag0s and Au0s adsorption sites on metal NPs to antibonding orbitals of the adsorbates have been identified for Ag/tpy, Ag/T-tpy, Au/tpy and Au/T-tpy nanoparticulate arrays. While the surface-adsorbate complexes displaying a photoinduced CT are known for Ag NPs, the Au0s surface complexes with this CT are newly reported. Bis(tpy)oligothiophenes were found to be attached to both Ag and Au NPs via the tpy group(s). The match between the interparticle distances within the NP islands and the lengths of the oligomers molecules indicates that the molecules act as interparticle linkers. In this case, an unequivocal spectral marker band evidence of the Ag0s as well as Au0s surface complex formation has not been obtained.

Introduction Plasmonic metal nanoparticles (NPs) and their ordered arrays attract great attention because of their unique optical properties originating from the physics of localized surface plasmon resonances (LSPR) 1–4, a coherent oscillation of the surface conduction electrons excited by electromagnetic (EM) radiation. The LSPR is highly affected by the size and shape of NPs and by the surrounding environment. Moreover, the LSPR can be tuned by spatial assembling of the NPs into arrays5-9, which are new materials with unique properties distinct from those of individual NPs, which enable their applications in opto1 ACS Paragon Plus Environment

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electronic management,10,11 enhancing the solar cell efficiency,12,13 chemi- and bio-sensors etc.14 A high variety of methods has been used for fabrication of ordered arrays, mainly nanolithography15,16 and physico-chemical methods such as controlled drying of sols of functionalized NPs17,18, electrophoretic deposition19, Langmuir-Blodgett technique20-22, self-assembling of NPs at both air-liquid23 and liquidliquid interfaces24-27, etching of Ag surfaces using various templates28 and others. The LSPR and morphology of nanocomposites (NCs) of metal NPs and organic ligands can be tuned by controlling: (i) the size and shape of NPs, (ii) the ligand structure, and (iii) conditions of assembling. All these factors influence the structural arrangement of NPs in arrays such as their periodicity, distribution of interparticle distances and the refractive index of the surrounding dielectric medium. The LSPR is responsible for the principle, so called electromagnetic mechanism (EM) of the surfaceenhanced Raman scattering (SERS) and other surface-enhanced processes.3-5,29,30 Besides, a chemical mechanism31,32 can complement the dominant EM mechanism if the wavelength of exciting radiation also obeys the resonance condition for excitation of a metal-to-(metal-adsorbate surface complex) or a metal-adsorbate surface complex exhibiting photoinduced charge transfer (CT) transition33-37. Lombardi et al. integrated the EM and CT processes of the SERS enhancement together with molecular resonances into a unified theory38. The CT transitions were proved for SERS experiments carried out on both roughened electrodes33,34,37,39 and plasmonic metal NPs and their assemblies36,40-42. In an electrochemical setup with a roughened electrode, the energy of CT transition can be tuned by the applied potential. On the other hand, in a system with plasmonic NPs, resonance conditions can be achieved only by a proper selection of the excitation wavelength. The overall enhancement of the Raman signal can be as high as 1012 if the adsorbate molecules are localized in hot spots, which allows detection of molecules on the single molecule level43-47. So high signal enhancement and the surface selectivity make the SERS spectroscopy a powerful tool in the chemical analysis and investigation of the surface-plasmon-mediated reactions. Two different adsorption sites on the surface of Ag36,39-41,48,49 or Au36,50 NPs were reported. In the SERS active Ag/adsorbate systems derived from Ag NPs stabilized by anions such as borate or citrate anions, the adsorbate molecules are predominantly bound to cationic adsorption sites that are usually labelled as Ag+ or Ag(I) sites. However, under specific conditions, namely upon a hydrosol pretreatment by halide anions39-41,48,49, a fraction of spectrally different surface complexes has been observed and assigned to adsorbate molecules coordinated to uncharged adsorption sites usually labelled as Ag0 or Ag(0) sites. The two spectrally different forms in Ag NPs/adsorbate systems were observed for 2,2´bipyridine (bpy),39,41 rhodamine48 and acridine.49 The surface complex species of the Ag0 type have been also observed for bpy adsorbed on a roughened Ag electrode39 and in assemblies formed at the interface between an Ag NP hydrosol and a dichloromethane solution of 2,2':6',2''-terpyridine (tpy),40 or under the strongly reducing conditions during Ag NP growth in the presence of bpy.41 Uncharged surface complex species was also observed for assembly of Au NPs with a strong electron-acceptor: tetracyanoquinodimethane known under abbreviation TCNQ.50 As to the formation of surface adsorbate-metal complexes with the CT transition, the highest attention has been payed to N-base molecules such as pyridine37, bpy39,51, tpy40, azobenzenes36, but rarely to S-base molecules such as 4-mercaptopyridine35. The Raman spectra of synthetically prepared inorganic metal complexes of adsorbates with or without metal-to-ligand charge-transfer (MLCT) or

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ligand-to-metal charge-transfer have been exploited in the SERS spectral analyses of surface adsorbate complexes40-42. Derivatives of tpy are utilized in the “bottom-up” self-assembly fabrication of nanocomposites. For these purposes, the metal NPs are functionalized by tpy derivatives carrying –SH52-54 or ionic groups,55 or the metal NPs are directly assembled with tetrakis(tpy) derivatives.56 The tpy-functionalized NPs are subsequently self–assembled with metal ions to provide ordered NC systems. Linear α,ω-bis(tpy) conjugated oligomers, which are in the spotlight as building blocks for electromagnetic-field-responsive metallo-supramolecular polymers57, represent another class of tpy derivatives that can be used for selfassembly construction of such NCs. In this work, we focus on the structure of nanocomposite monolayers (arrays) formed by spontaneous self-assembling at the interface between a hydrosol of Ag or Au NPs and a ligand solution in dichloromethane. 2,2´:6´,2´´-Terpyridine (tpy) and its 4´-derivatives containing thiophene ring(s): 4´-(2thienyl)-2,2´:6´,2´´-terpyridine (T-tpy), 5,5´-bis(tpy)-2,2´-bithiophene (tpy-2T-tpy), 5,5´´-bis(tpy)2,2´;5´,2´´-terthiophene (tpy-3T-tpy) are used as ligands (see Figure 1). Simple but efficient procedure for preparation of an array at a liquid-liquid interface and its facile transfer onto a glass slide or microscopic Cu grid is described in detailed. The arrays´ morphology is characterized by the transmission electron microscopy (TEM) images and advanced methods of the TEM image analysis. Correlation between the morphological and the optical characteristics of the arrays is established. Structures of the surfaceadsorbate complexes present in these arrays are identified with the aid of auxiliary model complexes and surface species derived from the studied ligands. The SERS and resonance Raman spectral measurements carried out with various excitation wavelengths are used as the main analytical tool.

Figure 1. Ligands (adsorbates) used for preparation of Ag- and Au arrays.

Experimental Materials Analytical-grade chemicals: methanol (Merck and UVASOL), FeSO4.5H2O (Lachema), Fe(ClO4)2.6H2O, NaOH, NH2OH·HCl, AgNO3, acetonitrile (all Sigma-Aldrich), NaBH4 and CH2Cl2 (both Merck), CHCl3 (Lachner); 2,2´:6´,2´´-terpyridine (tpy; Alfa Aesar) and distilled deionized water were used for sample preparations. Ligands (Figure 1): 4´-(2-Thienyl)-2,2´:6´,2´´-terpyridine (T-tpy); 5,5´-bis(tpy)-2,2´dithiophene (tpy-2T-tpy); 5,5´´-bis(tpy)-2,2´;5´,2´´-terthiophene (tpy-3T-tpy) were prepared by the procedures described earlier.58,59 3 ACS Paragon Plus Environment

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Preparation of hydrosols Ag NP hydrosol was prepared by reduction of silver nitrate by NH2OH·HCl according to the preparation procedure reported by Leopold and Lendl.60 Au NP hydrosol was prepared by reduction of HAuCl4.3H2O by sodium citrate according to the preparation procedure reported by Lee and Meisel.61

Preparation of nanocomposite arrays Nanocomposite arrays of Ag and Au NPs with studied ligands are further labeled according to the general format: Ag/ligand and Au/ligand, respectively. The arrays were prepared using the adopted preparation procedure reported earlier for other adsorbates.62,63 Briefly, a two-phase system consisting of 2 ml of Ag or Au hydrosol and 2ml of a dichloromethane solution of tpy (1 × 10-3 M) or T-tpy, tpy-2T-tpy, tpy-3T-tpy (all 1 × 10-4 M) was vigorously shaken for 2 minutes. When the shaking was stopped, a self-assembled nanoparticulate array appeared at spontaneously formed interface. The array layer was then collected together with a small portion of water phase by a 1 mL micropipette and transferred onto a glass slide, where it spontaneously reassembled into a nanoparticulate monolayer covering a water drop. For the UV/vis and Raman measurements, a liquid was slowly sucked of the drop by a paper and thus the dried array occurred deposited directly on the glass slide. For the TEM measurement, the array covering the water drop was touched by a carbon-coated Cu grid and thus “self-transferred” to the grid (see Figure 2).

Figure 2 – Scheme of the preparation and deposition of NPs arrays.

Preparation of auxiliary metal complexes The T-tpy complexes: [Ag(T-tpy)]NO3 and [Fe(T-tpy)2]SO4 were prepared by mixing stoichiometric amounts of T-tpy and the respective salt (AgNO3 or FeSO4) in methanol and isolated by a slow evaporation of the solvent. Complex [Au(T-tpy)]Cl3 was prepared by mixing stoichiometric amounts of Ttpy and HAuCl4 in acetonitrile solution and the solvent evaporation. Silver and gold complexes of ditopic ligands: [Ag2(tpy-2T-tpy)](NO3)2, [Au2(tpy-2T-tpy)]Cl3, [Ag2(tpy-3T-tpy)](NO3)2 and [Au2(tpy-3T-tpy)]Cl3 were prepared by mixing stoichiometric amounts of the particular ligand and salt in acetonitrilechloroform (1:1 by volume) mixed solvent (salts: AgNO3 and HAuCl4 were added as solutions in acetonitrile). Microcrystalline samples of the complexes were obtained by a slow solvent evaporation. Metallo-supramolecular polymers: P(tpy-2T-tpy/Fe) and P(tpy-3T-tpy/Fe) with perchlorate counterions were prepared by mixing stoichiometric amounts of the particular ligand and Fe(ClO4)2 in acetonitrilechloroform (1:1 by volume) mixed solvent and isolated by a slow solvent evaporation. 4 ACS Paragon Plus Environment

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Preparation of auxiliary hydrosols systems with model surface species Auxiliary hydrosols systems: Ag(sol)/T-tpy and Au(sol)/T-tpy were prepared by adding a methanol solution of T-tpy (1 µL, 2.5×10-3 M) into 1 mL of a hydrosol Ag NP or Au NP, respectively. Hydrosol systems Ag(sol)/[Fe(T-tpy)2]2+ and Au(sol)/[Fe(T-tpy)2]2+ were prepared similarly by adding a solution of [Fe(T-tpy)2]SO4 (5 µL, 5×10-4 M) into 1 mL of the given hydrosol.

Instrumentation Raman spectra of all kinds (normal, resonance (RR), SERS and SERRS(surface-enhanced resonance Raman scattering)) were recorded on a DXR Raman spectrometer (Thermo Scientific) interfaced to an Olympus microscope, employing an objective 50x for the solid samples; a macro-adapter for liquid samples was used for measurements on hydrosols. The 445 nm (diode laser), 532 nm (diode-pumped solid-state laser), 633 nm (He–Ne laser) and/or 780 nm (diode laser) excitation lines were used. The laser power ranged from 0.1 to 2 mW for the solid state samples and from 10 to 24 mW for solutions. The full-scale grating was used for all measurements. The UV/vis absorption and surface plasmon extinction (SPE) spectra were measured on a Shimadzu UV-2401 spectrometer. Transmission electron microscope images were obtained with a Tecnai G2 (FEI) transmission electron microscope with the acceleration voltage 120 keV.

Results and discussion SPE spectra and morphology of arrays The TEM micrographs shown in Figures 3 demonstrate two-dimensionality of the arrays assembled at the hydrosol-dichloromethane interfaces and transferred to supports by the procedure depicted in Figure 2. The SPE spectra of the studied Ag/ligand and Au/ligand arrays deposited on glass supports are displayed in Figure 4 together with the spectra of the parent hydrosols of non-aggregated NPs. As can be seen, the SPE bands of arrays are strongly red shifted with respect to the band of the corresponding sol of non-aggregated NPs, the shift being much higher for Ag arrays than for Au arrays. The highest red shift of the SPE band is observed for the smallest ligand tpy (∆λE = 330 nm for Ag/tpy; 267 nm for Au/tpy), smaller for T-tpy ligand (270 nm for Ag/T-tpy; 222 nm for Au/T-tpy) and the smallest for large ditopic ligands tpy-2T-tpy and tpy-3T-tpy (∆λE = 200 ± 10 nm for Ag- and 175 ± 3 nm for Au-arrays). Shoulders localized at 420 nm for Ag-arrays and at 550 nm for Au-array should be attributed to the quadrupolar plasmon resonance mode of the given metal NPs, since its position is not a function of the ligand.64-66

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Figure 3. TEM micrographs of the studied Ag/ligand and Au/ligand arrays deposited on carbon coated Cu grids (TEM micrographs of larger areas are shown in Figure S1 in the Supporting Information). Inset in the image of Au/tpy array shows fused Au NPs.

Figure 4. Normalized SPE spectra of Ag/ligand and Au/ligand arrays deposited on glass supports; dashed curves show the spectra of hydrosols of non-aggregated Ag- resp. Au-NPs and dashed verticals the Raman excitation lines. The size and spatial (areal for arrays) distributions of plasmonic NPs are the main factors controlling the surface plasmon resonance of NPs in their array and thus also the position of the SPE band of the 6 ACS Paragon Plus Environment

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array.67 The TEM images displayed in Figures 3 and S1 visually show how the distribution of NPs in selfassembled arrays changes with the type of both ligand and NPs. For arrays with Ag NPs, the distribution changes from nearly uniform (Ag/tpy), through non-uniform but still continuous (Ag/T-tpy) to a discontinuous composed of isolated islands of NPs (Ag/tpy-2T-tpy and Ag/tpy-3T-tpy). For arrays with Au NPs, the NP distribution of the Ag/T-tpy array seems to be more uniform than that of the Ag/tpy array. In addition, the Au/tpy array contains fused Au-NPs (see inset in Figures 3 and S2-a) that are rare or not observed in other here studied Au-arrays. We have examined that the fused Au NPs are reproducibly formed also in Au/tpy arrays obtained from Au NPs hydrosol stabilized with borate anions that was prepared using the procedure described in ref.27 (Figure S2-b). Quantitative analysis of the TEM images using the NIS-Elements imaging software provided the values of the morphology parameters defined below (Figure S3): a) average particle size ; b) average inter-particle distance of all NPs in the array, ; c) average inter-particle distance of “close-spaced particles” (CSP), , which is calculated excluding the distances exceeding certain cut-off limit (typically 4-5 nm);7 d) occupied area fraction, σOc (the area fraction occupied by plasmonic NPs); e) average zone-of-influence diameter, , where dZl is the diameter of a sphere (circle) that represents a zone defined by mid-distance points of the given NP to the surfaces of all nearneighboring NPs. This parameter is a measure of the periodicity of near-neighboring particles. All parameters were determined for Ag arrays while, in the case of Au arrays, the above-mentioned fusion of Au NPs prevented determining the parameters and for the Au/tpy array. The determined values of the above morphology-related parameters are summarized in Table S1 in the Supporting Information and their correlations with λSPE are shown in Figure 5. Plots in Figure 5 indicate: (i) dependence of λSPE on parameters , σOc and , and (ii) independence of λSPE on parameters and . Parameters (average inter-particle distance) and σOc (occupied area fraction) are closely related quantities: the higher is , the lower is σOc. The average zone-of-influence diameter is a measure of the array periodicity, which can be estimated by the less laborious method: discrete Fourier transformation analysis (DFTA) described below. The TEM micrograph analysis by the DFTA method in the reciprocal space was performed as follows (Figure S4): (i) TEM micrograph was segmented and binarized using the freeware program ImageJ68, (ii) the 2D discrete fast Fourier transforms of the binarized image was calculated using our own program that is described elsewhere69, and (iii) the latter image was radially averaged to obtain the 1D-radial profile (Figure S5). This profile corresponds to a calculated powder diffraction pattern which is analogue of the diffraction pattern that might be obtained from small-angle scattering measurements.69 Therefore, a peak or shoulder on a one-dimensional (1D) radial profile corresponds to an average periodic distance in the analyzed structure. The peak intensity is proportional to the structure regularity

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and the peak position is related to the average periodic distance in the array (Periodicity = 1/Peak position × Real width of image).69

Figure 5. Morphologic parameters of the Ag/ligand (left) and Au/ligand (right) arrays as a function of the SPE band position λSPE; NPs is the average diameter of NPs; average zone-of-influence diameter of NPs; average inter-particle distance; average inter-particle distance in CSP island (for cut-off distance of 4 nm); σOc the occupied area fraction (multiplied by factor of 50). The DFTA results (Figure S5) showed a decrease in the array regularity with the increasing size of the ligand molecule for all Ag/ligand arrays and Au/ligand arrays derived from the ligands containing thiophene ring(s). The Au/tpy array does not obey this trend showing the regularity as low as the Au/tpy3T-tpy array. The average periodic distance among NPs could be reliably estimated from the DFTA 1Dradial profiles only for the most regular arrays: ∼30 nm for Ag/tpy and ∼37 nm for Au/T-tpy. These values are essentially equal to those of the average zone-of-influence diameter (29 nm for Ag/tpy and 37 nm for Au/T-tpy arrays, see Table S1) which is the parameter that is the most laboriously determined by the NIS-Elements imaging software. Gong et al. 7, studied correlation between λSPE and the above morphologic parameters on arrays of Ag NPs prepared by the gas-phase deposition. They have reported strong correlation of λSPE with σOc (same as we do here), and a good correlation of λSPE with which they finally claimed as the crucial parameter controlling the λSPE value, as it reflects the near-field coupling of NPs. However, they did their measurements on arrays of almost evenly distributed Ag NPs for which the parameters and are equal and σOc is closely correlated with . In contrast, our measurements have been done on arrays of unevenly distributed plasmonic NPs and the results obtained clearly show the absence of the significant correlation between λSPE and . According to our analyses, the value of λSPE mainly depends on the difference - which corresponds to the size of CSP islands. The lower is this

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difference, the bigger are the CSP islands and the higher is the extent of delocalization of surface plasmon and thus also the displacement ∆λSPE of the SPE band and vice versa.

Surface complexes in Ag/tpy and Au/tpy arrays The SERS spectra of Ag/tpy array (Figure 6-A) taken with different excitation wavelengths (λexc) are practically identical to the spectra published earlier.40 The spectra excited at 532 nm are almost identical to the SERS and RR spectra of [Fe(tpy)2]2+ complex in which the MLCT, i.e., Fe(II) → π*(tpy) CT transition takes place. As the MLCT requires an electron transfer from a metal atom to the ligand π* orbital, this spectral pattern had been attributed to tpy ligand coordinated to uncharged silver atoms, i.e., to the surface species Ag0s -tpy.40 In contrast, the spectrum excited at 780 nm is identical to the spectrum of [Ag(tpy)]NO3, ref.40 Therefore, the spectral pattern for λexc = 780 nm had been attributed to tpy ligand 40 coordinated to Ag+ ions, further denoted as Ag s -tpy. The spectrum excited at 633 nm represents

transition between the above two spectra. In summary, spectra of Ag/tpy array confirm the already described coexistence of the ionic and non-ionic surface species in these arrays40 and thus also reproducibility of formation of surface species Ag0s -tpy.

Figure 6. SERS spectra of Ag/tpy and Au/tpy arrays.

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The SERS spectra of Au/tpy array (Figure 6-B) are similar to the spectra of Ag/tpy array but they show (i) much less pronounced overtones at 2700 - 3200 cm-1, and (ii) more bands in the region of breathing modes at around 1000 cm-1 (for λexc = 633 and 780 nm), which indicates the presence of non-coordinated pyridine rings. As only adsorbed tpy species form the array, the bands of unbound pyridine rings should originate from tpy species bound to Au NPs irregularly via one or two pyridine rings. Nevertheless, the most important observation is that the SERS spectrum of Au/tpy excited at 532 nm is nearly identical (except for overtones´ intensity) to the spectra of Ag/tpy and [Fe(tpy)2]2+ species, which both exhibit the CT transition. This strongly indicates the presence of species with the MLCT transition: Au0s -tpy, in Au/tpy array. To our best knowledge, this is the first published observation of the photoinduced CT transition between Au NPs and the adsorbed tpy ligand.

Figure 7. SERS spectra of Ag/T-tpy and Au/T- tpy arrays.

Coordination of T-tpy in arrays with Ag and Au nanoparticles Regarding detection of species with MLCT in Ag and Au/tpy arrays, the question has arisen as to the presence of such species in the arrays derived from T-tpy and other ligands with tpy groups. The SERS spectra of Ag/T-tpy (Figure 7-A) are relatively good resolved and show similar changes with λexc as the Ag/tpy array. In contrast, the spectra of Au/T-tpy array (Figure 7-B) are much less resolved and less depend on λexc. However, the presence of the overtone bands in SERS spectra excited at 532 nm 10 ACS Paragon Plus Environment

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indicates the presence of the MLCT species in both arrays, but mainly in the Ag/T-tpy array, regarding deep changes in its spectral pattern due to the change in λexc. In order to identify how T-tpy molecules are coordinated to metal NPs, several auxiliary T-tpy complexes and sol system with and without the MLCT were prepared and investigated. As a complex with the MLCT transition was chosen salt [Fe(T-tpy)2]SO4 which shows a strong MLCT absorption band at 576 nm (Figure S6).58 Raman spectra of this complex depend strongly on λexc (Figure S7). The spectra excited at 780 nm are off resonance spectra while those excited at 532 and 633 are RR spectra since these lines match the left (532 nm) respectively right (633 nm) arm of the MLCT band. The spectrum excited at 532 nm shows enormously enhanced bands of coordinated tpy units (in cis conformation) and a series of their overtones and combined bands that are absent in the spectra excited at 633 and 780 nm. A significant difference between the spectra excited at 532 and 633 nm proves that these lines match bands of different symmetry. SERS active surface species with the MLCT transitions: Ag(sol)/[Fe(T-tpy)2]2+ and Au(sol)/[Fe(T-tpy)2]2+ (Figure S8), were prepared by adding [Fe(T-tpy)2]SO4 to hydrosols of Ag and Au NPs. Raman spectra of both sol systems depend on λexc similarly as the spectra of [Fe(T-tpy)2]SO4 (Figures 8 and S8). The SERRS spectra (λexc = 532 nm) of sol systems are almost identical to the RR spectrum of [Fe(T-tpy)2]SO4 (see Figures 8 A-C left). The band positions of the sol systems differ by 1 cm-1 between themselves and up to 5 cm-1 when compared to the bands of [Fe(T-tpy)2]SO4 salt. Overtones and combination bands are also observed with a good wavelength agreement. These facts indicate preserved integrity of [Fe(T-tpy)2]2+ species upon their adsorption on Ag or Au NPs. The spectra excited at 780 nm (Figures 8 A-C right) reveal a good agreement of spectral patterns of Ag(sol)/[Fe(T-tpy)2]2+ and [Fe(T-tpy)2]SO4, which proves physisorption of [Fe(T-tpy)2]2+ species on Ag NPs70 without their coordination via thiophene ring. On the other hand, the SERS spectrum of Au(sol)/[Fe(T-tpy)2]2+ differs from the other two spectra mainly by the stretching mode band at 1385 cm-1. Considering preserved coordination of tpy groups to Fe2+ ions (otherwise there would not be strongly enhanced tpy bands in the spectra excited at 532 nm), this band shall be assigned to the [Fe(T-tpy)2]2+ species chemisorbed on Au NPs by coordination via sulfur atom of thiophene ring. Well known high affinity of sulfur atoms to Au0 atoms supports this explanation. Ionic T-tpy complexes without MLCT: [Ag(T-tpy)]NO3 and [Au(T-tpy)]Cl3, were prepared by mixing equimolar amounts of T-tpy and AgNO3 in methanol, respectively, T-tpy and HAuCl4 in acetonitrile, and isolated by solvent evaporation. No absorption band is observable in the region 500 – 800 nm in the UV/vis spectra of these complexes (Figure S6). Accordingly, Raman spectra of the [Ag(T-tpy)]+ and [Au(Ttpy)]3+ complex ions do not depend on λexc and do not show resonantly enhanced tpy bands. Moreover, Raman spectra excited at 532 nm were overlaid by a strong fluorescence. The spectra excited at 780 nm (Figure 8 H, I right) show nearly equal positions of almost all bands. This similarity indicates coordination of T-tpy molecules to Au3+ ions via tpy groups in [Au(T-tpy)]3+ species, which is in accord with the data in ref. 71

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Figure 8. Comparison of the Raman spectra of Ag/tpy and Au/T-tpy arrays with the spectra of auxiliary complexes and hydrosols; left: λexc = 532 nm; right: λexc = 780 nm. The SERS active sol systems without MLCT were prepared by adding a methanol solution of T-tpy into non-aggregated hydrosols of Ag and Au NPs. The SERS spectra of Ag(sol)/T-tpy system (Figure 8) do not change with λexc and are nearly identical to the spectra of [Ag(T-tpy)]+NO3. This indicates that they originate from the ionic species [Ag(T-tpy)]+ located at surfaces of Ag NPs, which we denote as + Ag s -tpy-T, to emphasize that T-tpy is coordinated via tpy group to an Ag surface site. The spectrum of

Au(sol)/T-tpy system excited at 780 nm significantly differs from that of [Au(T-tpy)]3+. This spectrum dominates the band at 1385 cm-1 observed also for Au(sol)/[Fe(T-tpy)2]2+ (vide supra), which thus can be in analogy assigned to T-tpy species bound to Au NPs via thiophene ring. These species we denote as Aux+ s -T-tpy to emphasize the coordination mode of T-tpy and uncertainty in the species charge (x+). The band at 1385 cm-1 dominates also the spectrum of Au(sol)/T-tpy system excited at 532 nm (Figure 8), which does not show any feature typical of the species with the MLCT (which requires coordination via tpy groups). Coordination modes in Ag/T-tpy arrays. A comparison of the Raman spectra of Ag/T-tpy and Au/T-tpy arrays with the spectra of auxiliary T-tpy compounds allows estimation of the coordination modes of Ttpy molecules in these arrays. The spectrum of Ag/T-tpy array excited at 532 nm exhibits the best match

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with the spectra of [Fe(T-tpy)2]2+ and Ag(sol)/[Fe(T-tpy)2]2+ but substantially differs from the spectrum of Ag(sol)/T-tpy (Figure 8). This proves the presence of the surface species with MLCT transitions in Ag/Ttpy array. Such species should be T-tpy molecules coordinated to the surface Ag0 atoms via tpy groups, i.e., species Ag0s -tpy-T. The spectrum of Ag/T-tpy array excited at 780 nm, in contrast, exhibits almost precise match with the spectra of [Ag(T-tpy)]+ and Ag(sol)/T-tpy (Figure 9), which do not show the MLCT transitions. This match proves the presence of the ionic surface species Ag+s -tpy-T in Ag/T-tpy arrays. The spectrum of Ag/T-tpy excited at 633 nm actually exhibits features of both these surface species. In summary, the spectral data prove the presence of at least two kinds of surface complex species in Ag/Ttpy array: the ionic Ag+s -tpy-T species without MLCT and the neutral Ag0s -tpy-T species with MLCT. Regarding the strong CT resonance enhancement (~102) of the SERRS signal of Ag0s -tpy-T for λexc = 532 nm and the absence of the SERS signal of this form in the spectra excited at 780 nm, the ionic surface species Ag+s -tpy-T should dominate in Ag/T-tpy arrays. Coordination modes in Au/T-tpy arrays. The SERS spectrum of Au/T-tpy array excited at 780 nm is similar to the spectrum of Au(sol)/T-tpy system (Figures 8 E, G right), which indicates the presence of Aux+ s -T-tpy species in Au/T-tpy. On the other hand, the spectrum excited at 532 nm exhibits, besides the -1 band of the Aux+ s -T-tpy species (1385 cm ) also the features typical of the species with MLCT transitions,

in particular weak band at 1537 cm-1 and combined bands and overtones at above 2600 cm-1 (Figure 8-E, left). This indicates the presence of the surface species with T-tpy molecules coordinated to Au0 atoms through tpy groups: Au0s -tpy-T. In addition, the band at 1385 cm-1 evidently overlaps the bands apparent in the spectrum of ionic species with T-tpy molecules bound to Aux+ ions via tpy groups. Therefore, the Aux+ s -T-tpy species should be also present in the Au/T-tpy array and thus also coordination of some T-tpy molecules via both tpy group and thiophene ring is probable.

Coordination of α,ω-bis(tpy)oligothiophenes in arrays with Ag and Au nanoparticles The SERS spectra of these nanocomposites (Figures 9, S9 and S10) substantially differ from Raman spectra of free tpy-2T-tpy and tpy-3T-tpy oligomers57 and, unlike the case of free oligomers, a fluorescence background was not observed in collected spectra. These differences indicate chemisorption of bis(tpy) species on metal NPs. The presence of different coordination modes in these NC arrays has been again studied using related model systems with and without MLCT. The set of Raman excitation lines has been extended to 445 nm because both free bis(tpy)oligothiophenes absorb light also at this wavelength (Figure S11). As model compounds with MLCT transitions were chosen linear metallo-supramolecular polymers P(Fe2+/tpy-2T-tpy) and P(Fe2+/tpy-3T-tpy) (UV/vis spectra are shown in Figure S12) which have been prepared by mixing respective oligomers with Fe2+ ions to the mole ratio of 1:1.72 Raman spectra of these coordination polymers (Figure S13) strongly depend on λexc. Excitation at 445 nm gives spectra with resonantly enhanced bands of thiophene rings. Excitation at 532 nm gives spectra with resonantly enhanced bands of tpy groups that are typical of tpy species of the MLCT type. The spectra excited at 13 ACS Paragon Plus Environment

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633 nm partly show enhanced tpy bands at 1290 and 1366 cm-1 and the spectra excited at 780 nm are off-resonance spectra.

Figure 9. SERS spectra of Ag/tpy-2T-tpy and Ag/tpy-3T-tpy arrays and auxiliary salts with: [Ag2(tpy-2Ttpy)]2+ and [Ag2(tpy-3T-tpy)]2+ ions (up) and SERS (λexc = 780 nm) and RR (λexc = 445 nm) spectra of Au/tpy-2T-tpy and Au/tpy-3T-tpy arrays and auxiliary salts with complex ions: [Au2(tpy-2T-tpy)]6+ and [Au2(tpy-3T-tpy)]6+ (down).

Model ionic complexes without MLCT transitions have been prepared by mixing a solution of tpy-2Ttpy or tpy-3T-tpy with a solution AgNO3 or HAuCl4 in the metal ions/oligomer mole ratio of 2:1 (i.e., 1:1 for tpy unis). Raman spectra of these ionic complexes showed a fluorescence background that was possible to remove at data processing. The spectra of Ag+ complexes almost do not depend on λexc, while the spectra of Aux+ complexes show uniformity only for λexc of 532, 633 and 780 nm; the spectrum excited at 445 nm shows strong resonantly enhanced bands of thiophene rings (band at ca 420 nm, Figure S12) and attenuated bands of tpy groups. Coordination modes in Ag/bis(tpy)oligothiophene arrays. The SERS spectra of these arrays (Figure 9) match the spectra of ionic Ag+ complexes and also show a good agreement with the spectra of Fe2+ polymers (Figure S13), but only for λexc = 445 and 780 nm. The spectra excited at 532 nm (resp. 633 nm) 14 ACS Paragon Plus Environment

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surprisingly do not show the spectral features typical of tpy species with MLCT transitions: strongly enhanced bands of tpy groups – 1366, 1407, 1539 and 1563 cm-1. Thus these spectra indicate that only ionic species with bis(tpy)oligothiophene ligands bound to the surface metal ions via tpy groups: Ag+s -tpy-2T-tpy and Ag+s -tpy-3T-tpy are present in these arrays. Coordination modes in Au/bis(tpy)oligothiophene arrays. Raman spectra of these arrays excited at 445 nm are simple RR spectra, since Au arrays do not exhibit significant SPE at 445 nm (Figure 4b). As this excitation matches absorption band of thiophene rings (Figure S11), the bands of breathing (994 cm1

) and stretching (1470 cm-1) modes as well as overtones (2930 cm-1) observed in these spectra should

mainly originate from oligothiophene blocks (Figure 9). The SERS spectra excited at 532, 633 and 780 nm differ from the RR spectra mainly by attenuated (532 nm) or absent band of the overtones and increased intensity of the bands of tpy groups. Their spectral patterns are similar to the patterns of model ionic complexes and, in addition, the features typical of tpy species with MLCT transitions are absent in these spectra. All significant SERS spectral bands of Au/bis(tpy)oligothiophene arrays can be assigned to the species with ligands coordinated to Au ions via tpy end-groups. In addition, the position of the thiophene ring breathing-mode band in the RR spectra (994 cm-1) is typical of uncoordinated rings. This indicates coordination of bis(tpy)molecules to the Au NP surface ions via tpy end-groups, leaving thiophene rings non-coordinated. Coordination of bis(tpy) molecules via thiophene rings (vide supra) thus seems improbable in these arrays while coordination of bis(tpy) species as bridges connecting neighboring metal NPs is not excluded. As it is evident from preceding paragraphs, similarity of spectral patterns and agreements in the band positions play the key role in performed analyses. It is thus worth mentioning that the observed minor differences between spectral patterns of related species should stem from different ionic (atomic) radii and coordination geometry of central atoms – octahedral for Fe2+, tetrahedral for Ag+ and planar for Au3+ ions.73 Moreover, this geometry need not be necessarily preserved on surfaces of NPs due to steric reasons. Full agreement of spectral patterns thus cannot be expected. Nevertheless, the spectral (non)conformities found and exploited in the above spectral analyses can be regarded as sufficiently conclusive.

Conclusions TEM images of the prepared arrays have conclusively shown that self-assembling of Ag and Au NPs with various tpy-type ligands, which takes place at the interface between the Ag or Au NP hydrosol and the ligand solution in dichloromethane, results into formation 2-D nanocomposite arrays and that these arrays can be easily transferred onto various substrates using the here described simple technique exploring the self-assembly capability of the arrays. Investigation of the relationship between the morphology and the optical responses of the arrays has revealed that the SPE band position depends on the size of island of CSPs rather than on CSP or . This result indicates that the SPE band position is governed by the extend of the delocalization of the surface plasmon excitation.

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SERS spectral probing of Au/tpy arrays at excitation wavelengths in the 445-780 nm spectral region 0 has revealed the coexistence of the cationic Aux s -tpy and the neutral (non-ionic) Aus -tpy surface

species, and a photoinduced Au0 → tpy CT is newly reported for the latter species. The SERS spectral study on Ag/tpy arrays confirmed the recently revealed40 coexistence of the Ag 0s -tpy surface complex (with the photoinduced CT transition) and of the Ag +s -tpy surface species in these arrays. Surface-adsorbate bonding in arrays with the bifunctional adsorbate T-tpy that possesses different coordination sites (tpy group and thiophene ring) is more complex. In Ag/T-tpy arrays, both the ionic 0 Ag s -tpy-T and non-ionic Ags -tpy-T surface species have been identified, both with T-tpy molecules

coordinated exclusively via tpy groups. SERS spectra of Au/T-tpy arrays revealed the presence of surface species with T-tpy molecules coordinated via tpy group to Au0 atoms (species with the photoinduced CT transition) and Aux+ ions and, unlike the Ag/T-tpy array, the species with T-tpy molecules coordinated via sulfur atom of thiophene ring. Bridging of Au NPs by T-tpy molecules as linkers has also to be considered, since it could actually explain the highest uniformity of the Au/T-tpy array of all the Au NP arrays studied therein. Analogously to the T-tpy molecules, molecules of bis(tpy)oligothiophenes could possibly coordinate to Au NPs through tpy groups as well as thiophene rings. However, the obtained spectral data did not provide any evidence for coordination via thiophene rings. This fact is not surprising as it can be easily explained by the steric effects of near-neighboring groups, which should make coordination of 2,5disubstituted thiophene rings to Au adsorption sites difficult or at least unstable. Both bis(tpy)oligothiophenes were found to be attached to both Ag and Au NPs via the tpy group(s). Since the interparticle distances within the NP islands are well comparable with the lengths of the oligomer molecules, it is quite probable that at least a fraction of the molecules acts as interparticle linkers within NPs in CSP islands. In this case, the SERS spectra and their excitation wavelength dependence have not provided an unequivocal spectral marker band evidence of the Ag0s as well as Au0s surface complex formation, since the SERS spectral patterns in the 950-1100 cm-1 region are substantially more complicated in the case of the highly conjugated molecular interconnects than for the more simple molecules. Moreover, no progress in the intensity of spectral bands in the 1530-1560 cm-1 region analogous to RR spectra of P(Fe2+/tpy-3T-tpy) and P(Fe2+/tpy-2T-tpy) was observed.

Acknowledgement: This work was supported by the Czech Science Foundation (P108/12/1143 and 15-22305S), the Grant Agency of Charles University (project 363515) and the COST Action CM1302 (SIPS) European Network on Smart Inorganic Polymers. Supporting Information Available: Values of morphological related parameters obtained by analyses of TEM images with NIS-Elements software and by the discrete two-dimensional fast Fourier transform, SPE, UV/vis and Raman spectra of ligands studied assemblies and auxiliary systems.

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Table of contents:

Nanoparticulate Ag/ligand and Au/ligand arrays self-assembled at interfaces between metal hydrosols and dichloromethane solutions of terpyridine-based ligands exhibit the ligand-dependent morphology and optical responses controlled by the size of islands of closely spaced plasmonic nanoparticles. Besides dominating ionic surface species, the arrays contain neutral species exhibiting photoinduced metal-toligand charge transfer transitions. Absence of such species in arrays with ditopic ligands is explained by dynamic processes in nanoparticulate systems with bridge-coordinated ligands.

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